Wireless devices increasingly use complex modulation schemes, such as phase-shift-keying (PSK) to modulate voice, data, or other input information. For example, to support higher data rates, GSM-based wireless devices now support 8-PSK modulation scheme to modulate a signal that may convey the input information. Regardless, such wireless devices use power amplifiers to amplify signals, which then can be transmitted using an appropriate antenna.
Among other design constraints, power amplifiers should be designed to avoid distortion particularly at the extremes of the dynamic range of such amplifiers. Linearly modulated signals have dynamic range requirements that, among other things, depend upon the number of codes (users). Peaks or nulls may occur in such wireless devices when individual carriers (codes) are phase aligned. With the increasing number of carriers or codes being used in such devices, the probability of peaks and nulls occurring has gone up in such devices. This places high peak-power requirements on the power amplifiers used in such wireless devices.
To reduce the peak-power requirements on a power amplifier, typically either the average power or peak-to-average ratio may be lowered. Often, the wireless system link budget determines the average power, and thus the average power cannot be lowered that easily. Accordingly, when the average power cannot be changed, one possibility is to reduce the peaks, i.e., reduce the peak-to-average ratio, using measurement based systems, such as feed-back or feed-forward systems.
Such systems, however, typically require complex measurements associated with either determining an appropriate feed-back signal or a feed-forward signal, which then may be fed to the power amplifier. Further, such techniques may result in adverse effects on a wireless device's system parameters, such as bit-error rate and/or vector error. Accordingly, there is a need for systems, methods, and apparatus for reducing dynamic range requirements of a power amplifier in a wireless device.